1. Field of the Invention
The disclosure relates generally to the field of magnetic disk drives, and more particularly to apparatus and methods related to testing disk drive components.
2. Description of the Prior Art
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. A typical magnetic disk drive comprises a head, including a slider and a transducer, in very close proximity to a surface of a rotatable magnetic disk. The transducer, in turn, includes a write element and/or a read element. As the magnetic disk rotates beneath the head, an air bearing is formed between the surface of the magnetic disk and an air bearing surface (ABS) of the slider. The air bearing causes the head to “fly” above the surface of the magnetic disk with a separation (“fly height”) on the order of micro-inches or less. As the head flies over the magnetic disk, the write element and the read element can be alternately employed to write and read data bits along a magnetic “track” on the magnetic disk.
Briefly, the write element operates by passing an electric current through a coil wound around a core, or “yoke,” to induce a magnetic field therein. In longitudinal recording applications, the yoke is shaped like a horseshoe with a very narrow gap between the two ends. The gap is filled with low magnetic saturation material. Accordingly, in longitudinal recording applications, the magnetic field induced by the electric current must bridge the gap without going through the material within the gap and does so by expanding outward (“fringing”) beyond the ABS and to the disk. Data may be written where the magnetic field interacts with the disk.
Unfortunately, generating the magnetic field in the yoke also generates heat that can cause the coil to expand. This expansion produces a force that can cause the write element to distort and protrude beyond the ABS. This effect is known as pole tip protrusion (PTP) and is deleterious because it can decrease the spacing between the head and the disk to the point where the head might contact the magnetic disk. It should be noted that although write elements for perpendicular recording applications are arranged differently than those for longitudinal recording, perpendicular recording write elements are also susceptible to pole tip protrusion for much the same reasons. Even if the head-disk contact is not severe (so-called “soft contact”) the contact can affect the alignment between the head and the track and can lead to read/write errors. More severe contacts (so-called “hard contacts”) can cause additional read/write errors and/or damage the magnetic recording head transducer. However, the pole tip protrusion due to write-induced heating typically exists only temporarily and on a microscopic scale. Therefore, it has been very difficult to evaluate its effect on the alignment between the head and the disk.
Accordingly, what is needed is a practical method for evaluating misalignment between the head and the track caused by pole tip protrusion.